<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
    <input type="text" id="i1">
    <input type="text" id="i2">
    <input type="button" value="加" onclick="f(1)">
    <input type="button" value="减" onclick="f(2)">
    <input type="button" value="乘" onclick="f(3)">
    <input type="button" value="除" onclick="f(4)">
    <div>
        结果:<span>0</span>
    </div>
    <script>
        function f(x) {
            //得到页面中的相关元素对象
            let i1 = document.querySelector("#i1");
            let i2 = document.querySelector("#i2");
            let s = document.querySelector("span");

            //判断输入的两个值是不是NaN
            if (isNaN(i1.value) || isNaN(i2.value)) {
                s.innerText = "输入错误!";
                return;
            }

            switch (x) {
                case 1: //加
                    s.innerText = i1.value*1 + i2.value*1;
                    break;
                case 2: //减
                    s.innerText = i1.value - i2.value;
                    break;
                case 3: //乘
                    s.innerText = i1.value * i2.value;
                    break;
                case 4: //除
                    s.innerText = i1.value / i2.value;
                    break;
            }
        }
    </script>
</body>
</html>